Packet radio is a digital radio communications mode. Earlier digital modes were telegraphy (Morse Code), teleprinter (Baudot) and facsimile. Like those earlier modes, packet was intended as a way to reliably transmit written information. The primary advantage was initially expected to be increased speed, but as the protocol developed, other capabilities surfaced.
By the early 1990s, packet radio was recognized as a way not only to send text, but also to send files (including small computer programs), handle repetitive transmissions, control remote systems, etc.
The technology itself was a leap forward, making it possible for nearly any packet station to act as a digipeater, linking distant stations with each other through ad hoc networks. This makes packet especially useful for emergency communications. In addition, mobile packet radio stations can automatically transmit their location, and check in periodically with the network to show that they are still operating.
The most common use of packet is in amateur radio, to construct wireless computer networks. Packet radio uses the AX.25 (Amateur X.25) data link layer protocol, derived from the X.25 protocol suite and adapted for amateur radio use. AX.25 was developed in the 1970s and is based on the wired network protocol X.25. AX.25 includes a digipeater field to allow other stations to automatically repeat packets to extend the range of transmitters. One advantage is that every packet sent contains the sender’s and recipient’s amateur radio callsign, thus providing station identification with every transmission.
Aloha and PRNET
Since radio circuits inherently possess a broadcast network topology (i.e., many or all nodes are connected to the network simultaneously), one of the first technical challenges faced in the implementation of packet radio networks was a means to control access to a shared communications channel. Professor Norman Abramson of the University of Hawaii developed a packet radio network known as ALOHAnet and performed a number of experiments around 1970 to develop methods to arbitrate access to a shared radio channel by network nodes. This system operated on UHF frequencies at 9600 baud. From this work the Aloha multiple access protocol was derived. Subsequent enhancements in channel access techniques made by Leonard Kleinrock et al. in 1975 would lead Robert Metcalfe to use carrier sense multiple access (CSMA) protocols in the design of the now commonplace Ethernet local area network (LAN) technology.
Over 1973-1976, DARPA created a packet radio network called PRNET in the San Francisco Bay area and conducted a series of experiments with SRI to verify the use of ARPANET (a precursor to the Internet) communications protocols (later known as IP) over packet radio links between mobile and fixed network nodes. This system was quite advanced, as it made use of direct sequence spread spectrum (DSSS) modulation and forward error correction (FEC) techniques to provide 100 kbit/s and 400 kbit/s data channels. These experiments were generally considered to be successful, and also marked the first demonstration of Internetworking, as in these experiments data was routed between the ARPANET, PRNET, and SATNET (a satellite packet radio network) networks. Throughout the 1970s and 1980s, DARPA operated a number of terrestrial and satellite packet radio networks connected to the ARPANET at various military and government installations.
Amateur Packet Radio and the AMPRNet
Amateur radio operators began experimenting with packet radio in 1978, when – after obtaining authorization from the Canadian government – Robert Rouleau, VE2PY, Norm Pearl, VE2BQS, and Jacques Orsali, VE2EHP of the Montreal Amateur Radio Club Montreal, Quebec began experimenting with transmitting ASCII encoded data over VHF amateur radio frequencies using homebuilt equipment. In 1980, Doug Lockhart VE7APU, and the Vancouver Area Digital Communications Group (VADCG) in Vancouver, British Columbia began producing standardized equipment (Terminal Node Controllers) in quantity for use in amateur packet radio networks. In 2003, Rouleau was inducted into CQ Amateur Radio magazine’s hall of fame for his work on the Montreal Protocol in 1978.
Not long after this activity began in Canada, amateurs in the US became interested in packet radio. In 1980, the U.S. Federal Communications Commission (FCC) granted authorization for U.S. amateurs to transmit ASCII codes via amateur radio. The first known amateur packet radio activity in the US occurred in San Francisco during December 1980, when a packet repeater was put into operation on 2 meters by Hank Magnuski KA6M, and the Pacific Packet Radio Society (PPRS). In keeping with the dominance of DARPA and ARPANET at the time, the nascent amateur packet radio network was dubbed the AMPRNet in DARPA style. Magnuski obtained IP address allocations in the 18.104.22.168 network for amateur radio use worldwide.
Many groups of amateur radio operators interested in packet radio soon formed throughout the country including the Pacific Packet Radio Society (PPRS) in California, the Tucson Amateur Packet Radio Corporation (TAPR) in Arizona and the Amateur Radio Research and Development Corporation (AMRAD) in Washington, D.C.
By 1983, TAPR was offering the first TNC available in kit form. Packet radio started becoming more and more popular across North America and by 1984 the first packet based bulletin board systems began to appear. Packet radio proved its value for emergency operations following the crash of an Aeromexico airliner in a neighborhood in Cerritos, California Labor Day weekend, 1986. Volunteers linked several key sites to pass text traffic via packet radio which kept voice frequencies clear.
For an objective description of early developments in amateur packet radio, refer to the article “Packet Radio in the Amateur Service”.
The most common use of packet radio today is in amateur radio, to construct wireless computer networks. Its name is a reference to the use of packet switching between network nodes. Packet radio networks use the AX.25 data link layer protocol, derived from the X.25 protocol suite and adapted for amateur radio use.